Cytomegalovirus is a species-specific virus belonging to the Herpesvirus family. This DNA virus causes severe disease or even death in newborns and immunosuppressed patients. Murine cytomegalovirus provides an opportunity for studying the virus in the natural host. Furthermore, the MCMV genome can be manipulated to identify viral genes which regulate pathogenesis. Towards this goal, we have identified four open reading frames (ORFs m138, M139, M140 and M141) whose products are nonessential for virus replication in fibroblasts. However when ORFs m142 and m143 are deleted in addition to the four neighboring ORFs, MCMV is unable to grow in the absence of wild type virus. Therefore, m142 and/or m143 are likely essential genes. ORF m142 and m143 are transcribed with immediate early kinetics and are members of the US22 gene family, some of which are transcriptional transactivators. The objective of this study is to identify the essential gene(s) which is required for growth of MCMV in fibroblasts, to characterize the gene product(s), and to determine its ability to function as a transcriptional transactivator of viral genes important in MCMV DNA replication.